Many systems use light emitting diodes (LEDs) to generate light. For example, LEDs are often used in display devices to generate red, green, and blue light. Each color could be generated using one or more strings of LEDs, where each string can include multiple LEDs coupled in series. Often times, LED strings are driven sequentially, where one string is turned on and off and then the next string is turned on and off (usually without overlap).
Conventional devices operating in this manner can include a power converter that uses a fixed reference voltage to generate a fixed output voltage for the LED strings. Conventional devices can also include a linear regulator for each LED string. The linear regulator compares (i) a reference voltage for its LED string and (ii) a sense voltage generated by a sense resistor coupled in series with the LEDs in its string. The linear regulator typically controls a pass transistor coupled in series with the LEDs in its string.
This approach, however, is not particularly efficient. Linear regulators often require a voltage overhead so that the pass transistor operates in a gain region. Also, the voltage regulator generates a fixed output voltage regardless of the LED string being driven. These and other issues can cause large power losses. The worst case efficiency of a typical LED driver could be around 65%.